The present invention relates to method and apparatus for filtering particles from the air, and more particularly to an electrostatic filter including a material which catches the air particles and other impurities that flow through the system.
Many contaminants are present in air. These contaminants include dust particles, odors, viruses, hair, bacteria, etc. These contaminants when inhaled can cause illnesses such as asthma, colds and the flu. Also present in the air is radon. Radon contains charged particles that attach to dust particles. These particles may be inhaled into the body and adsorbed into the lungs. When the radon particles decay, they emit radiation which may alter or destroy cells and cause cancer. Accordingly, it is desirable to remove these contaminants from the air.
Numerous methods and apparatuses are known and utilized to remove air borne contaminants from air streams. One such apparatus is an electric precipitator which involves a two-step process to remove the particles suspended in the air stream. In the first step, the particles pass through an electric discharge area to ionize the air. The ions so produced collide with the suspended particles and confer on them an electric charge.
In the second step in this process, these charged particles are precipitated on electrode plates that have a high voltage gradient imposed therebetween. Efficiency of these electric precipitators is limited by the resistance of the dust to be collected, the area of the collector plate relative to the volume of air cleaned, and the amount of charge on the dust to be collected. Dust particles to be precipitated must generally have resistivities between 5.times.10.sup.3 and 2.times.10.sup.10 ohms per centimeter. Outside this range, particles cannot be efficiently collected, which therefore significantly restricts the electric precipitator application. Further, electrostatic precipitators remove only particulate matter, and not objectionable gases. These apparatuses are also subject to the disadvantages that the charges applied to the surfaces of the belts are susceptible of being pulled off the belts by charged dust particles.
A further difficulty associated with this type of apparatus is that the particles are subject to reintrainment. Upon contacting the collection surface, the particles may either be neutralized or reversed in polarity depending on the strength of the charge imparted to the belt. In either event, they are susceptible to being dislodged from the belt surface by motion of the gas stream, or by the attractive forces of the oppositely charged dust carried by gas stream, because the restrain of the belt primarily by non-electrical affects.
Moreover, the cleaning of the collection plates of the prior art systems presents a serious problem in so much as a substantial amount of the reintrainment of the dust occurs. Removal of the dust from the plates is normally accomplished by vibrating the collection plates to dislodge dust particles which fall by gravity into hoppers located beneath the plates. Because of the proximity of the plates to the gas flow channel, however, some of the dislodged dust particles are reintroduced into the gas stream. These particles must be recharged and again collected for effective removal from the stream. This necessitates a lengthening of the collection zone to compensate for reintrainment of the particles during the removal operation. Exemplary electric precipitators are disclosed in U.S. Pat. Nos. 2,579,440; 3,581,468; and 3,626,668.
Another type of apparatus frequently used to remove air borne contaminants is a filter, designed as an assembly containing very small obstacles such as fibers intricately bound together or loosely bound hygrogate through which the dirty air flows. The mechanical filter captures particles because the particles inertia and diffusion causes a collision with the filter media, although the collection efficiency for a large number of collectors in the typical filter medium is very high. Unfortunately, the large collection area in these mechanical filters also produces higher restrictions to air flow than electric precipitators. Restrictions on air flow reduces the rate at which air can be filtered. Further, these mechanical filters may not always be able to filter out small particles within a gas stream. Also, these filters must be frequently changed, as they fill up with dust particles.
An improvement in filter performance is realized by electrifying the filter medium to increase filter efficiency and filter life. These filters are based upon the concept of either charging or polarizing a filter medium generating an electric force between the medium and the particles.
Compared to a conventional filter, the electrofiberous filter has a much higher efficiency. When an external electric field is first applied to the filter medium, the capture mechanism is due to the forces between the polarized medium and the polarized or charged particles. The electric field instantly polarizes the filter medium, which then attracts both charged and polarized particles. Examplary electric filters are disclosed in U.S. Pat. Nos. 3,800,509; 3,375,638; 3,537,238; and 4,405,342.
The electric filters disclosed in the preceding patents have air that flows through the filter medium. This blockage of air flow can reduce the amount of time that the air filter may take to remove all the particles from a room. Further, non-charged particles may not be ionized and would thereby not be filtered. Further, these devices remove small particles but may not be able to remove the larger particles from the air. Finally, these devices may not be able to remove odors from the air without restricting the air flow.
One such device that is used to collect particles larger than a pre-selected size from a particle laden air stream is disclosed in U.S. Pat. No. 4,182,673. This device uses an inertial impaction to separate particles larger than a specified size from smaller particles in an air stream. The device collects these particles on a moving adhesive collection surface. With this device, the collection surface moves in a direction opposing the air stream. A drawback of this device is that the air stream speed may have to be lowered so that the particles will adhere to the collection surface. Another drawback of this device is that odors in the gases that pass through the device will not be eliminated. Further, only large particles may be removed from this device and not the smaller particles.